1. Field of the Invention
This invention relates to hydrocracking or hydroprocessing systems and processes that employ ebullated-bed reactors.
2. Description of Related Art
Common objectives of hydrocracking or hydroprocessing operations are to remove impurities such as sulfur, nitrogen and/or metals, particularly in residue feedstocks, and cracking the heavy feed into lower molecular weight hydrocarbons having lower boiling points to obtain transportation fuels such as gasoline and diesel. The reactions that occur in hydrocracking/hydroprocessing operations include hydrodesulfurization (HDS), carbon residue reduction (CRR), nitrogen removal (HDN), and cracking.
In a refinery, it is desirable to minimize the down-time for replacement or regeneration of catalyst. Further, process economics generally require a versatile system capable of handling feed streams of varying contaminants levels such as sulfur, nitrogen, metals and/or organometallic compounds, such as those found in a vacuum gas oils, deasphalted oils and residues.
There are three common reactor types used in the refining industry: fixed, ebullated, and moving bed. The decision to utilize a particular type of reactor is based on a number of criteria including the characteristics of the feedstock, desired conversion percentage, flexibility, run length, and requisite product quality.
The ebullated-bed reactor was developed to overcome plugging problems commonly associated with fixed-bed reactors during processing of relatively heavy feedstocks and as the conversion requirements increases, e.g., for vacuum residue. In an ebullated-bed reactor, the catalyst is in an expanded bed, thereby countering plugging problems associated with fixed-bed reactors. The fluidized nature of the catalyst in an ebullated-bed reactor also allows for on-line catalyst replacement of a small portion of the bed. This results in a high net bed activity which does not vary with time.
Fixed-bed technologies have considerable problems in treating particularly heavy charges containing relatively high quantity of heteroatoms, metals and asphaltenes, as these contaminants cause the rapid deactivation of the catalyst and plugging of the reactor. Multiple fixed-bed reactors connected in series can be used to achieve a relatively high conversion of heavy feedstocks boiling above 370° C., but such designs require high capital investment and, for certain feedstocks, commercially impractical, e.g. catalysts replacement every 3-4 months.
Therefore, to treat these heavy charges, ebullated-bed reactors were developed and are in operation worldwide. These reactors have numerous advantages in performance and efficiency, particularly with heavy crudes. Early ebullated-bed processes and systems are described by Johanson in U.S. Pat. Nos. 2,987,465 and 3,197,288, both of which are incorporated herein by reference.
In general, an ebullated-bed reactor includes concurrently flowing streams of liquids, solids and gas through a vertically-oriented cylindrical vessel containing catalyst. The catalyst is placed in motion in the liquid and has a gross volume dispersed through the liquid medium that is greater than the volume of the catalyst mass when stationary. Ebullated-bed reactors are incorporated in various refinery operations, including processes for the upgrading of heavy liquid hydrocarbons and the conversion of coal to synthetic oils.
Typically, a liquid hydrocarbon phase and a gaseous hydrogen phase are passed upwardly through the bed of catalyst particles at a rate such that the particles are forced into motion as the fluids pass upwardly through the bed. The catalyst bed expansion level is, at least in part, determined by the bottoms recycle liquid flow rate, which is controlled by an ebullating-bed pump. During steady-state operation (i.e., the ebullated-bed state), the bulk of the catalyst does not rise above a certain level in the reactor which is predetermined during reactor design. This level is established to prevent the catalyst particles from leaving the reactor or to interfere with the efficient operation of the cyclones that are installed inside the reactors to separate carried-over catalyst particles from the gas-liquid mixture. More catalyst can be loaded into the reactor initially because of no gas hold-up and liquid viscosity. These design criteria are well within the routine skill in the art. A substantial portion of the product vapors and liquids pass through the upper level of the catalyst particles into a substantially catalyst-free zone and are removed proximate to the upper portion of the reactor.
Substantial amounts of hydrogen gas and light hydrocarbon vapors rise through the reaction zone into the catalyst-free zone. Liquid is recycled to the bottom of the reactor and removed from the reactor as net product from this catalyst-free zone. A certain portion of the vapor is separated from the liquid recycle stream before being passed through the recycle conduit drawn by suction of ebullating pump. However, gases or vapors present in the bottoms recycle stream materially decrease the capacity of the recycle pump. The presence of vapors also reduces the liquid residence time in the reactor and limit hydrogen partial pressure.
Certain reactors employed in the catalytic hydrocracking process with an ebullated-bed of catalyst particles are designed with a central vertical recycle conduit which serves as the downflow conduit, or downcomer, for recycling liquid from the catalyst-free zone above the ebullated catalyst bed to the suction of a recycle pump to re-circulate the liquid through the catalytic reaction zone.
FIG. 1 schematically illustrates a system and apparatus 100 of the prior art in which liquid is recycled internally with a recycle downflow conduit. Apparatus 100 includes an ebullated-bed reactor 160 and an ebullating pump 164. Ebullated-bed reactor 160 includes an inlet 130 for receiving a mixture of hydrogen gas and feedstock and an outlet 166 for discharging product effluent. Ebullating pump 164 is in fluid communication with the ebullated-bed reactor 160 and includes an inlet 162 for receiving effluent recycled from ebullated-bed reactor 160 and an outlet 163 for discharging the recycled effluent at an increased pressure.
In the practice of system 100, a mixture of hydrogen gas and feedstock is introduced into the ebullated-bed reactor 160 via inlet 130 for reaction that includes conversion of the feedstock into lower molecular weight hydrocarbons. Liquid reaction effluent continuously flows down in the downflow conduit located inside ebullated-bed reactor 160, and is recycled back to the ebullated-bed reactor 160 at elevated pressure using ebullating pump 164. Product effluent is recovered via outlet 166.
Alternatively, the recycle liquid can be obtained from a vapor separator located downstream of the reactor or obtained from an atmospheric stripper bottoms stream. The recycling of liquid serves to ebullate and stabilize the catalyst bed, and maintain temperature uniformity through the reactor.
FIG. 2 schematically illustrates a system and apparatus 200 of the prior art with an external recycle system that includes an ebullated-bed reactor 260, an ebullating pump 264 and a high-pressure separator 280. Ebullated-bed reactor 260 includes an inlet 230 for introducing a mixture of hydrogen gas and feedstock and an outlet 266 for discharging product effluent. High-pressure separator 280 includes an inlet in fluid communication with outlet 266 for receiving product effluent, an outlet 282 for discharging a gas product stream and an outlet 284 for discharging a liquid stream. Ebullating pump 264 includes an inlet 262 in fluid communication with outlet of high-pressure separator for receiving at least a portion of the liquid stream, and an outlet 263 for discharging recycling stream at elevated pressure.
In the practice of system 200, a mixture of hydrogen gas and feedstock is introduced into the ebullated-bed reactor 260 via inlet 230 for reaction which includes the conversion of the feedstock into lower molecular weight hydrocarbons. Reaction effluent is conveyed to the high-pressure separator 280 to obtain a gas stream 282 and a liquid stream 284. At least a portion of the liquid stream 284 is recycled as stream 288 to the ebullated-bed reactor 260 via ebullating pump 264. The remaining portion of the stream 284 can be recovered as product stream 286 or subjected to further refinery processes.
Catalyst bed expansion is an important factor in the ebullated-bed reactor. In the process, the expansion of the bed is controlled by the recycle pump speed. Certain systems include a bed characterized by a number of bed level detectors and one or more additional detectors for determining abnormally high bed (interface) level. The interface level is detected, for instance, by a density detector including a radiation source at an interior point within the reactor and/or a detection source in the reactor wall.
Although ebullated-bed processes are generally used for conversion of heavier vacuum residue feedstocks, they can also be used to clean or treat a lower boiling point vacuum gas oil feedstock. Advantages of ebullated-bed processes include product quality and rate uniformity, reduced downtime and lower capital investment.
The volume and length-to-diameter ratio are known to be factors in ebullated-bed reactor design that impact the catalyst load. For a given volume reactor, the greater the length-to-diameter ratio, the more catalyst that can be introduced into the reactor.
Gas and liquid hold-up rates are important process characteristics that contribute to the system performance. High gas hold-up rates result in decreased liquid residence time which lowers process performance. The gas hold-up rate in an ebullated-bed reactor can be as high as 40%.
Although there are numerous types of ebullated-bed reactor designs, the problems exists of providing a more efficient and effective ebullated-bed reactor process and achieving improved reactor performance that eliminates or minimizes gas hold-up in the recycle system and its adverse effect on the recycle pump.